1. Technical Field
The present invention relates to a driving technique for a discharge lamp lit by an electric discharge between electrodes.
2. Related Art
A discharge lamp such as an extra-high pressure mercury lamp or a metal halide lamp is used as a light source for an image display apparatus such as a projector. The discharge lamp is driven by, for example, a driving method for supplying a high-frequency alternating current. With the driving method, it is possible to stabilize an electric discharge, prevent so-called blackening, devitrification, and the like of a discharge lamp main body, and suppress a decrease in the life of the discharge lamp (e.g., JP-A-2007-115534).
As another driving method for the discharge lamp, there is a driving method for supplying a low-frequency alternating current having a rectangular waveform (a square wave alternating current). With the driving method, when the discharge lamp is lit, since protrusions are formed and grow at distal end portions of a pair of electrodes, it is possible to maintain a narrow electrode interval state (e.g., JP-A-2010-114064).
In light emission of the discharge lamp in the alternating-current driving, plasma density in the vicinity of the pair of electrodes changes according to positive and negative polarity switching of the alternating current. The change in the plasma density appears as a decrease or an increase in internal gas density, changes to vibration, and is propagated from the vicinity of the pair of electrodes to an inner wall. When the vibration is reflected on the inner wall and returns to the vicinity of the pair of electrodes, the vibration and the reflected vibration sometimes intensify each other according to the resonance phenomenon. A part of coil sections provided in the electrodes is damaged or the electrodes are broken by the resonance phenomenon. A basic frequency at which the resonance phenomenon occurs is referred to as an acoustic resonance frequency. The resonance phenomenon occurs not only at the acoustic resonance frequency but also at a frequency fc/2n (n is a natural number) when the acoustic resonance frequency is represented as fc. The amplitude of the vibration decreases as n increases.
In the driving method for supplying the high-frequency alternating current, when the discharge lamp is lit, the electrodes are heated by an arc discharge that occurs between the electrodes. The electrodes melt and the distance between the electrodes gradually increases. Further, the driving method for supplying the high-frequency alternating current is easily affected by the resonance phenomenon compared with the driving method for supplying the low-frequency alternating current.
On the other hand, with the driving method for supplying the low-frequency alternating current, blackening, devitrification, or the like of the discharge lamp main body occurs and the life of the discharge lamp decreases.
In the driving method for supplying the high-frequency alternating current, when the discharge lamp is lit, the electrodes are heated by an arc discharge that occurs between the electrodes. The electrodes melt and the distance between the electrodes gradually increases. On the other hand, with the driving method for supplying the low-frequency square wave alternating current, blackening, devitrification, or the like of the discharge lamp main body occurs and the life of the discharge lamp decreases.
Therefore, a driving method was attempted in which the supply of the high-frequency alternating current and the supply of the low-frequency square wave alternating current were combined. However, various problems occurred in the driving method.